ContainsMouse gives incorrect value on parent change - qt

Question

In QML, the MouseArea's containsMouse property is supposed to return true when the mouse is currently inside the mouse area. Unfortunately, this is not always the case. In the following code, the red square turns blue when the MouseArea within it contains the mouse (ContainsMouse is true). However, if you hit the control key while the square is blue, when the square is reparented to the Window's contentItem, the containsMouse property is not updated (as indicated by the text in the middle of the square). The square will still be blue even though it doesn't contain the mouse anymore. Is there anyway to tell the MouseArea to refresh it's containsMouse property?
Here is the code:
import QtQuick 2.12
import QtQuick.Window 2.12
Window {
width: 800
height: 500
visible: true
Rectangle {
id: square
width: 200
height: 200
focus: true
color: my_mouse_area.containsMouse ? "blue" : "red"
MouseArea {
id: my_mouse_area
anchors.fill: parent
hoverEnabled: true
onClicked: {
my_mouse_area.x = 200
}
}
Text {
anchors.centerIn: parent
text: my_mouse_area.containsMouse + ""
font.pixelSize: 20
}
Keys.onPressed: {
if(event.key === Qt.Key_Control){
second_window.show()
square.parent = second_window.contentItem
}
}
}
Window {
id: second_window
width: 400
height: 400
visible: false
}
}

Answer 1

I don't like my first solution, so I have made another, more sophisticated one, but this is not a pure QML solution. The trick is that on parent change you should call a C++ method where you send a mouse move event back to the mouse area, so it will re-evaluate the hovered aka containsMouse boolean. It is a nicer solution, but still a bit of a workaround.
Make sure you have a simple QObject derived class like MyObject with the following Q_INVOKABLE method:
class MyObject : public QObject
{
Q_OBJECT
//
// constuctor and whatnot
//
Q_INVOKABLE void sendMouseMoveEventTo(QObject* item)
{
QEvent* e = new QEvent(QEvent::MouseMove);
QCoreApplication::sendEvent(item, e);
}
};
Make an instance of it in main.cpp, and set as context property, so you can reach it from QML:
MyObject myObject;
engine.rootContext()->setContextProperty("myObject", &myObject);
And finally in the QML Rectangle add this:
onParentChanged: {
myObject.sendMouseMoveEventTo(my_mouse_area)
}

Answer 2

The solution I came up with uses Timer, but with zero interval, thus zero flickering. You can try setting the interval to higher value, to see what is going on. The trick is to set the rectangle visibility dependent of the timer running using "visible: !tmr.running", and start the timer immediately after the parent change of the rectangle.
import QtQuick 2.12
import QtQuick.Window 2.12
Window {
width: 800
height: 500
visible: true
Rectangle {
id: square
width: 200
height: 200
focus: true
color: my_mouse_area.containsMouse ? "blue" : "red"
visible: !tmr.running
Timer {
id: tmr
interval: 0
}
MouseArea {
id: my_mouse_area
anchors.fill: parent
hoverEnabled: true
onClicked: {
my_mouse_area.x = 200
}
}
Text {
anchors.centerIn: parent
text: my_mouse_area.containsMouse + ""
font.pixelSize: 20
}
Keys.onPressed: {
if(event.key === Qt.Key_Control){
second_window.show()
square.parent = second_window.contentItem
tmr.start()
}
}
}
Window {
id: second_window
width: 400
height: 400
visible: false
}
}